Skip setting default value as empty array for geospatial-indexed fields

Fix for workaround mentioned in [this SO discussion](http://stackoverflow.com/questions/16388836/does-applying-a-2dsphere-index-on-a-mongoose-schema-force-the-location-field-to/17328860?noredirect=1)

- Skips setting default value as empty array when both of the following occur:
    * Field is defined to have a 2dsphere or 2d index
    * No value is provided for the field
    * No default value is defined for the field in the schema
- If a field with a geospatial index is not assigned a value and the document is saved, on retrieval, the field in the document is  `undefined`

Issue: #1668
[Alternate solution](https://github.com/LearnBoost/mongoose/pull/1779)